I hope you're not being serious that it was us Hungarians who added the "A" in front of "ngola", to create the name of a Portuguese colony in Africa.The origin of the word comes from "ngola" which was the name of kings who ruled the Ndongo kingdom.
As it is not customary to start a word with two consonants in Hungarian and to help the pronunciation, a vowel was necessary.
The Hungarian being an agglutinative laguage, the last vowel of the word Angola is perceived as part of the noun itself and kept unchanged, unlike e.g. in Slavic languages where it is seen as a nominative singular ending, thus it changes according to the corresponding declension pattern (Angola, od Angoly, v Angole, s Angolou, ...). So the adverb angolul cannot derive from the noun Angola, but only from the adjective angol.... does angolul mean both "Angolanly" and "Englishly"? What are other words?
Is it common to say e.g. "tancovat maďarsky/česky/rusky" (to dance Hungarian/Czech/Russian) in Czech language?... For example let's say that there is an African dance which is danced differently in Angola and in Namibia. In Czech you can say "Dance it Angolanly, not Namibianly!". Is it possible in Hungarian?
In my opinion, the word angolaiul is grammatically and linguistically perfectly correct. It's a totally different question if there really exists a language called "angolai" (= "Angolian").But there are couple of Google results for "angolaiul" ...